Air Source Heat Pumps

On this page you will find helpful information on air source heat pumps:


Why Are Air Heat Pumps So Common?

 Air source heat pumps (ASHP's) are the most common heat pumps used in domestic and commercial buildings. This is because air heat pumps are easier to install and can be installed independently in some apartments, whereas this cannot be achieved with ground and water source heat pumps.

You can however have a thermal energy heating (ground) or Hydronic heating cooling (water) source heat pump plant that serves the whole building just like air source heat pumps and air-conditioning plants.

Air heat pumps are also less expensive to purchase and to install than ground or water source heat pumps, but these costs, as with all heat pump types, are quickly recovered compared to traditional non-renewable energy types of heating.

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How Efficient Are Air Heat Pumps?

The efficiency of air source heat pumps to provide heat, as with all types of heat pumps, far exceed traditional types of heating systems (see heat exchange process). However, the efficiency of the air heat pump is below that of the ground and water source heat pump due to the fluctuation of the external air temperature and is also reduced in moments of reduced air-flow and high wind conditions, unlike the more constant ground and water temperature.

The performance of air source heat pumps is again reduced in extended periods of below freezing temperature. Generally when external temperatures fall below 4.4oC a panel of electric resistance coils comes into play to provide indoor space heating (this is less efficient).

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Can Air Heat Pumps Provide Central Heating?

Yes, air heat pumps are also available as multi-split systems. These are heat pump systems for 'central heating' enabling you to have up to 8 indoor units coupled to only 1 outdoor unit. These systems eliminate the ducting required, as with gas fired forced air systems.

There are also air heat pump systems with 'Reverse Cycle Chillers' (RCC). Having this type of heat pump will enable you to have more heating and cooling distribution options. As well as having a multi-split system you can also have underfloor heating. This is possible because the heat pump is connected to a tank of water that's heavily insulated which the heat pump heats or cools.

Heat pumps with an RCC system can use the stored energy in the water to either heat or cool the air and allow the heat pump to work in low outdoor temperatures without the use of an electric resistance coil (heat pumps with RCC system are substantially more efficient than standard air source heat pumps).

The stored heat also eliminates the periodic blowing of cool air usual to air heat pumps during their defrost cycle and their initial start of the heating cycle. The hot water can also be used for underfloor heating.

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Things To Consider When Choosing An Air Heat Pump

  • Choose a heat pump that has defrost control. Basically this reverses the heat flow to defrost the external coil which will reduce the supplementary electrical energy use for heating by the heat pump.
  • Remember that fans and compressors make noise. If at all possible try to locate the outside unit away from windows (and from any windows in adjacent buildings). Noise can be further reduced by mounting the external unit on a sound absorbing base. All external units should have a sound rating - try selecting units that have a sound rating of 76 decibels or lower.
  • Make sure that the outdoor unit is protected from high winds as this will affect the efficiency of the heat absorption process. The right fencing and planting can achieve this. There are in development 'Cold Climate Heat Pumps' that have a two speed two-cylinder compressor, and a back-up booster compressor. These features allow this system to operate efficiently in temperatures as low as -9oC.
  • If you are going to use the heat pump more in the heating mode rather than the cooling mode then installing the indoor unit in a low wall position is better as this will increase its heating efficiency. However, if you are using the heat pump more for cooling, then install the unit high on the wall as this position will increase its cooling efficiency.
  • There are slim-lined indoor heat pump units available. They can be recessed into the wall to make them less intrusive in the room.

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What Are The Advantages of Air Heat Pumps?

  • Cheaper to purchase than other heat pumps such as ground and water source heat pumps
  • Easy install - can be installed in apartments
  • More energy efficient than traditional non-renewable energy systems
  • Reverse-Cycle - has a heating and cooling mode like all geothermal heat pumps
  • 'Reverse Cycle chillers' capable heat pumps offer additional heating and domestic hot water options
  • Energy savings - reduced annual fuel bills

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What Are The Disadvantages of Air Heat Pumps?

  • Air source heat pumps are not as efficient as ground and water source heat pumps
  • The performance and efficiency of air source heat pumps can be affected by;
    • Sustained temperatures below freezing
    • High or no wind conditions
  • Noise - some systems are quieter than others
  • Can be intrusive - Slim-Lined models are available

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